Effects of size and phase of TiO₂ in poly (vinyl alcohol)-based gel electrolyte on energy storage ability of flexible capacitive supercapacitors

Flexible supercapacitor (FSC) with high power density and cycle life has been considered as efficient energy storage device for soft electronics. Gel electrolyte is used in FSC to avoid electrolyte leakage. Titanium dioxide (TiO₂) with high dielectric constant and abundant surface vacant sites is regarded as efficient inorganic filler in gel electrolyte. In this work, TiO₂ nanoparticles are incorporated in polyvinyl alcohol/H₃PO₄ gel electrolyte for FSC. Different particle sizes and phases of TiO₂ are firstly used as inorganic filler in gel electrolyte to enhance ionic conductivity via reducing crystallinity of polymer. The FSC with P25 TiO₂ in gel electrolyte presents the best energy storage ability due to the smallest particle size and preferable phases. The highest specific capacitance (C_F) of 69.2 F/g at 20 mV/s is obtained for FSC with optimized P25 TiO₂ amount in gel-electrolyte. This FSC also shows the maximum energy density of 5.9 Wh/kg at 300 W/kg, and C_F retention of 94% after 3500 charging/discharging cycles. This work demonstrates feasibility of TiO₂ as efficient filler in gel electrolyte. It is worthy to study the relation between intrinsic property of TiO₂ and energy storage ability of FSC in other gel-electrolyte systems.